1. Field of the Invention
The present invention relates to a semiconductor device fabrication and more particularly, to a formation method of an alignment mark used for detecting or measuring the pattern alignment in optical lithography, and a fabrication method of a semiconductor device using the formation method of the mark.
2. Description of the Prior Art
In recent years, the reduction projection exposure technique has been popularly used in photolithography processes for fabricating semiconductor devices with the progressing miniaturization of circuit patterns. In the photolithography processes, typically, pattern formation steps are repeated to constitute the layered structure of a semiconductor device in such a way that an upper-level pattern is overlaid to be aligned with a lower-level pattern. The lower-level pattern may be formed in, on, or over a semiconductor substrate.
The upper-level pattern needs to be aligned with the lower-level pattern at a specific alignment accuracy. To detect or measure the alignment or overlay error of the upper-level pattern with respect to the lower-level pattern, usually, alignment marks are additionally formed in, on, or over the semiconductor substrate.
FIGS. 1A to 1I show a conventional fabrication method of a semiconductor device, in which an alignment mark is used.
First, a first oxide layer 302 is formed on a main surface of a semiconductor substrate 301, as shown in FIG. 1A. The first oxide layer 302 serves as a first interlayer dielectric layer.
Next, a polysilicon layer 303 is grown on the first oxide layer 302. A patterned photoresist film 304 is formed on the polysilicon layer 303 thus formed by a photolithography process, as shown in FIG. 1B. Using the patterned photoresist film 304 as a mask, the underlying polysilicon layer 303 is then patterned to a specific shape by an etching process, forming a polysilicon wiring layer 305 on the first oxide layer 302. The photoresist film 304 is then removed. The state at this stage is shown in FIG. 1C.
Subsequently, a second oxide layer 306 is formed on the first oxide layer 302 to cover the polysilicon wiring layer 305, as shown in FIG. 1D. The second oxide layer 306 serves as a second interlayer dielectric layer.
A patterned photoresist film 307 is formed on the second oxide layer 306 by a photolithography process. As shown in FIG. 1E, this photoresist film 307 has windows 307A, 307B, and 307C uncovering the second oxide layer 306. Using the patterned photoresist film 307 as a mask, the first and second oxide layers 302 and 306 are selectively removed by an anisotropic etching process, forming a recess 308A and contact holes 308B and 308C. The photoresist film 307 is then removed. The state at this stage is shown in FIG. 1F.
The recess 308A, which is located at a corresponding position to the window 307A of the photoresist film 307, penetrates vertically the first and second oxide layers 302 and 306 to extend to the main surface of the substrate 301. The contact hole 308B, which is located at a corresponding position to the window 307B of the photoresist film 307, penetrates vertically the second oxide layer 306 only to extend to the underlying wiring layer 305. The contact hole 308C, which is located at a corresponding position to the window 307C of the photoresist film 307, penetrates vertically the first and second oxide layers 302 and 306 to extend to a region (not shown) formed in the main surface of the substrate 301.
The recess 308A has a square plan shape and is used for formation of an alignment mark. The contact hole 308B is used for electrical connection to the underlying polysilicon wiring layer 305. The contact hole 308C is used for electrical connection to the underlying region in the main surface of the substrate 301.
Further, an aluminum silicide (AlSi) layer 309 is formed on the patterned second oxide layer 306 to fill the contact holes 308B and 308C, as shown in FIG. 1G. The AlSi layer 309 is contacted with the polysilicon wiring layer 305 in the contact hole 308B and with the region in the main surface of the substrate 301 in the contact hole 308C. In the recess 308A, the AlSi layer 309 covers the bottom and side walls of the recess 308A. The recess 308A is not filled with the AlSi layer 309. A square recess 308D is formed on the AlSi layer 309 in the recess 308A.
Following this, a patterned photoresist film 310 is formed on the AlSi layer 309 by a photolithography process, as shown in FIG. 1H. To form an alignment mark 312A, a part 310A of the photoresist film 310 is located on the AlSi layer 309 in the recess 308D. The part 310A has a square plan shape smaller than that of the recess 308D.
The square-shaped part 310A of the photoresist film 310 left in the square-shaped recess 308A constitutes the alignment mark 312A. The relative position of the part 310A in the recess 308A indicates the alignment or overlay error of the patterned photoresist film 310 with respect to the patterned second oxide layer 306 (i.e., the contact holes 308A and 308B).
Specifically, as shown in FIG. 2, the alignment error of the patterned photoresist film 310 (i.e., the upper-level pattern) with respect to the patterned second oxide layer 306 (i.e., the lower-level pattern) is able to be known by measuring the distances X1, X2, Y1, and Y2 from the four sides of the part 310A of the photoresist film 310 to the corresponding opposite walls of the recess 308A of the second oxide layer 306. The measurement of the distances X1, X2, Y1, and Y2 is usually carried out using a proper measuring tool such as a Scanning Electron Microscope (SEM) and an optical microscope.
Subsequently, using the patterned photoresist film 310 as a mask, the underlying AlSi layer 309 is selectively removed by an anisotropic etching process, forming AlSi wiring lines 311B and 311C on the second oxide layer 306. The photoresist film 310 is then removed. The state at this stage is shown in FIG. 1I.
As seen from FIG. 1I, the wiring line 311B is contacted with and electrically connected to the polysilicon wiring layer 305 through the contact hole 308B. The wiring line 311C is contacted with and electrically connected to the region in the main surface of the substrate 301 through the contact hole 308C.
Because the part 310A of the photoresist film 310 exists in the recess 308D, as shown in FIG. 1H, a square-shaped part 309A of the AlSi layer 309 is left in the recess 308A, as shown in FIG. 1I. Therefore, the square-shaped part 309A of the AlSi layer 309 and the square-shaped recess 308A may be used as an alignment mark 312B.
The conventional fabrication method of a semiconductor device as described above has the following problem.
Specifically, the first and second oxide layers 302 and 306 are selectively removed during the anisotropic etching process for the recess 308A and the contact holes 308B and 308C. The main surface of the substrate 301 is uncovered in the recess 308A through this anisotropic etching process, as shown in FIG. 1F.
On the other hand, during the photolithography process for patterning the photoresist film 310 on the AlSi film 309 shown in FIG. 1H, the exposure light is set to be focused on the photoresist film 310 located on the second oxide layer 306. Further, the exposure light is set so that the part 310A of the photoresist film 310 located on the bottom of the recess 308D is included in a specific depth of focus of the exposure light.
However, in recent years, the total thickness of the first and second oxide layers 302 and 306 often becomes greater than the specific depth of focus of the exposure light. For example, a Metal-Oxide-Semiconductor field-effect transistor (MOSFET) is typically formed on the substrate 301 region, which is located below the polysilicon wiring lines 305. It is popular that the first oxide layer 302 has a thickness of 1 .mu.m to 1.5 .mu.m and that the contact holes 308B and 308C have a depth of approximately 0.6 .mu.m.
In this case, the part 310A of the photoresist film 310 is very difficult to be included in the specific depth of focus of the exposure light, together with the remaining parts of the film 310 on the AlSi layer 309, resulting in an incorrect shape of the part 310A of the photoresist film 310. This incorrectness in shape leads to incorrect detection or measurement of the pattern misalignment.